1. Field of the Invention
This invention relates to polyorganosiloxane elastomer compositions. More particularly, this invention relates to curable polyorganosiloxane elastomer compositions capable of being cohesively bonded to various substrates, including those coated with organic impurities such as oils and greases. The bonding is achieved by means of a novel two-component liquid adhesion promoter composition.
2. Description of the Prior Art
One of the desirable properties of polyorganosiloxane elastomers, also referred to as silicone elastomers, is the ability of these materials to resist degradation and/or solubilization in the presence of heat and a variety of chemicals. It is therefore understandable that silicone elastomers have been used as protective coatings on a variety of organic and inorganic substrates. A shortcoming of many silicone elastomer compositions, particularly those cured at room temperature by a platinum catalyzed hydrosilation reaction is their poor adhesion to some metals, particularly aluminum.
It is known to add various organosilicon compounds to curable polyorganosiloxane compositions, particularly silicone elastomers, to achieve cohesive bonding between the cured elastomer and an organic or inorganic substrate. For example. U.S. Pat. No. 4,460,739. which issued to Ashby on July 17, 1984, discloses using maleamidyl substituted trialkoxy-or triacyloxysilanes for this purpose.
The use of partial hydrolysis products of aliphatically unsaturated alkoxysilanes such as vinyltrialkoxysilanes, as adhesion promoters for compositions comprising a vinyl terminated polyorganosiloxane, an organohydrogensiloxane crosslinking agent and a platinum catalyst is taught in U.S. Pat. No. 4,311,739, which issued to Hardman and Dujak on Jan. 19, 1982.
Imparting adhesion to polyorganosiloxane compositions that are curable to silicone elastomers using a platinum-catalyzed hydrosilation reaction by incorporating certain silicon-bonded reactive groups such as ester, epoxy or trialkoxysilylalkyl group into the ethylenically unsaturated polyorganosiloxane reactant is described in U.S. Pat. No. 4,245,079, which issued to Matsumoto and Murai on Jan. 13, 1981. When the reactive group is trialkoxysilylalkyl, the ethylenically unsaturated polyorganosiloxane reactant exhibits repeating units of the general formula--OSi(CH.sub.3)[RSi(OR').sub.3 ], where R is alkylene and R' is alkyl.
The use as adhesion promoters of organohydrogensiloxanes containing at least two silicon bonded hydrogen atoms per molecule and an average of at least one unit of the formula (RO).sub.3 Si(CH2).sub.x (CH.sub.3)SiO and/or (RO).sub.3 Si(CH.sub.2) (CH.sub.3).sub.2 SiO.sub.1/2 where R represents methyl, ethyl or CH.sub.3 C(O)- and x is 2 or 3 is taught in U.S. Pat. No. 3,772.066, which issued to Clark and Hays on Nov. 13, 1973. The adhesion promoters are used in polyorganosiloxane compositions that are curable by a platinum catalyzed hydrosilation reaction.
British Pat. No. 1,524,790, which issued on Sept. 13, 1978, teaches using a bis(trialkoxysilyl)alkane such as 1,2- bis(trimethoxysilyl)ethane as a binding medium to increase the compressive strength of mortar and other inorganic building materials.
The prior art also teaches using mixtures of two or more organsilicon compounds to achieve cohesive bonding between silicone elastomers and various substrates. Adding the combination of (1) a polyorganosiloxane containing a hydroxy group and a vinyl radical and (2) an epoxysilane to a silicone elastomer composition that is curable by a platinum catalyzed hydrosilation reaction is taught in U.S. Pat. No. 4,087,585 that issued to Schulz on May 2. 1978. The disadvantage of this combination as an adhesion promoter is that the curable composition must typically be heated to temperatures of at least 100 degrees C. to overcome the inhibiting effect of the adhesion promoter on the curing reaction.
U.S. Pat. No. 4,525,400, which issued on June 25, 1985 to Supernant, teaches an adhesion promoter for one-package, moisture curable compositions containing alkoxysilanes and/or acyloxysilanes as the crosslinking agent. The adhesion promoter is a cyclic organosiloxane wherein the silicon atoms are bonded to a group of the formula--RSiOR', the same group present in the polyorganosiloxanes described in the aforementioned patent to Matsumoto et al. R' can represent either alkyl, as in the case of Matsumoto et al.. or acyl. The--RSiOR' group is part of an adhesion promoter that is present in addition to the principal reactants of the moisture curable composition. In the compositions of Matsumoto et al. this group is present on an ethylenically unsaturated polyorganosiloxane that reacts with an organohydrogensiloxane in the presence of a platinum hydrosilation catalyst to form the silicone elastomer.
The adhesion promoters of Supernant are prepared by the reaction of a cyclic alkylhydrogensiloxane with an ethylenically unsaturated trialkoxy- or triacyloxysilane such as vinyltrimethoxysiloxane. The data in the examples of this patent demonstrate that the inventive adhesion promoter does not effectively bond a moisture cured silicone elastomer to Alclad(R) aluminum after the elastomer has cured for 10 days under ambient conditions. Under these conditions only 10 percent of the elastomer initially in contact with the substrate remained on the substrate when the elastomer layer was peeled away from the substrate. This value increased to 90 percent when the elastomer was allowed to cure for 25 days under ambient conditions after being applied to the substrate. In no instance did this value reach 100 percent using an Alclad aluminum substrate.
Supernant compares one of his inventive cyclosiloxanes with the corresponding linear polydiorganosiloxane containing an average of 20 repeating units per molecule. The silicon atoms are bonded to the same groups as in the inventive compound. In many instances the peel strength values reported for the two compounds are similar.
The peel adhesion tests reported by Supernant were conducted in accordance with ASTM test method No. C-794. In accordance with this method the test surfaces of the substrate must be cleaned with "methyl ethyl ketone or similar solvent" followed by an optional cleaning with a detergent solution before being coated with the material to be evaluated. Up until now it has been found difficult to cohesive bond room temperature curable silicone elastomers, particularly those formed by the platinum catalyzed reaction between an ethylenically unsaturated polyorganosiloxane and an organohydrogensiloxane, if the substrate surface to be coated has not been previously cleaned with an organic solvent to remove organic impurities and contaminants such as oil and grease.
One objective of this invention is to provide adhesion promoters that cohesively bond cured silicone elastomers to inorganic substrates, particularly those coated with organic impurities.
A second objective of this invention is to provide room temperature curable polyorganosiloxane compositions that cohesively bond to inorganic substrates, particularly metals, in the cured form.